Inkjet printing systems frequently make use of an inkjet printhead mounted to a carriage which is moved back and forth across a print media, such as paper. As the printhead is moved across the print media, control electronics activate an ejector portion of the printhead to eject, or jet, ink droplets from ejector nozzles and onto the print media to form images and characters. An ink supply provides ink replenishment for the printhead ejector portion.
Some printing systems make use of an ink supply that is replaceable separately from the printhead. When the ink supply is exhausted the ink supply is removed and replaced with a new ink supply. The printhead is then replaced at or near the end of printhead life and not when the ink supply is exhausted. When a replaceable printhead is capable of utilizing a plurality of ink supplies, this will be referred to as a “semipermanent” printhead. This is in contrast to a disposable printhead, that is replaced with each container of ink.
A significant issue with semipermanent printheads is premature failure due to loss of proper pressure regulation. To operate properly, many printheads have an operating pressure range that must be maintained in a narrow range of slightly negative gauge pressure, typically between −1 and −6 inches of water. Gauge pressure refers to a measured pressure relative to atmospheric pressure. Pressures referred to herein will all be gauge pressures. If the pressure becomes positive, printing and printing system storage will be adversely affected. During a printing operation, positive pressure can cause drooling and halt ejection of droplets. During storage, positive pressure can cause the printhead to drool. Ink that drools during storage can accumulate and coagulate on printheads and printer parts. This coagulated ink can permanently impair droplet ejection of the printhead and result in a need for costly printer repair. To avoid positive pressure, the printhead makes use of an internal mechanism to maintain negative pressure.
Air present in a printhead can interfere with the maintenance of negative pressure. When a printhead is initially filled with ink, air bubbles are often present. In addition, air accumulates during printhead life from a number of sources, including diffusion from outside atmosphere into the printhead and dissolved air coming out of the ink referred to as outgassing. During environmental changes, such as temperature increases or pressure drops, the air inside the printhead will expand in proportion to the total amount of air contained. This expansion is in opposition to the internal mechanism that maintains negative pressure. The internal mechanism within the printhead can compensate for these environmental changes over a limited range of environmental excursions. Outside of this range, the pressure in the printhead will become positive.
One solution to the air accumulation problem has been the use of disposable printheads. The amount of ink associated with a disposable printhead can be adjusted to keep air accumulation below a critical threshold. When the amount of ink is small, this increases the cost of printing by requiring frequent printhead replacement. Alternatively, the ink container can be made large to reduce frequency of printhead replacement. However, large ink containers become problematic when the printing application is a compact desktop printer. An example of a system utilizing a disposable printhead, wherein a large ink supply is replaced each time the printhead is replaced, is described in U.S. Pat. No. 5,369,429.
Another solution to the air accumulation problem has been the use of air purge mechanisms to make semipermanent printheads viable. An example of an air purge approach is described in U.S. Pat. No. 4,558,326. Issues with purging systems include the added printer cost for the purge mechanism, the reliability problems associated with accommodating the ink that tends to be purged out with air, and the stranding of air in the ink ejectors of the printhead, and increase in maintenance requirements.
Another solution to air management in inkjet printheads has been in the form of air warehousing. Air generated during the life of the pen is stored in the printhead. This requires the printhead to be able to compensate for expansion of the stored air due to thermal and pressure variations, which necessitates additional size and complexity. This additional size constrains the printer by placing more mass on the carriage and requiring a larger carriage for the printheads. As more printheads are added to the carriage, this issue becomes even more important.
It is known to use unsaturated ink in filling ink supplies. Insofar as is known, however, unsaturated ink has not heretofore been employed in addressing the problem of air accumulation in ink jet printheads.